CA1328497C

CA1328497C - Engine-sensing draft control system with multiple feedback compensation mechanisms

Info

Publication number: CA1328497C
Application number: CA000562326A
Authority: CA
Inventors: Kamyab Aghai-Tabriz; Barry D. Batcheller
Original assignee: Case LLC
Current assignee: CNH Industrial America LLC
Priority date: 1987-09-08
Filing date: 1988-03-24
Publication date: 1994-04-12
Anticipated expiration: 2011-04-12
Also published as: US4846283A

Abstract

ENGINE-SENSING DRAFT CONTROL SYSTEM
WITH MULTIPLE FEEDBACK COMPENSATION MECHANISMS
ABSTRACT OF THE DISCLOSURE

An automatic feedback control system for use with an off-road vehicle having an adjustable-elevation, ground-engaging implement, such as an agricultural tractor with a three-point hitch and a plough or cultivator, is disclosed.
The automatic control system provides an engine-sensing draft control system which adjusts the height or elevation of the ground-engaging implement so as to maintain the draft or load forces experienced by the vehicle at a substantially constant level. The determination of the draft being produced by the ground-engaging implement is determined by the engine speed deviation from a target point speed which is indicated by an operator-set desired no-load engine speed and an operator-set engine lug-down value or load. To provide proper operation of the engine-sensing draft control system, multiple feedback compensation mechanisms are provided, including a throttle compensation function, an electrohydraulic valve compensation function, a slip-sensing function, a sensitivity (deadband) adjustment, a maximum implement-lowering rate function, and an automatic downshift signal function to cause the transmission of the tractor to downshift to a lower speed to avoid engine stall under certain conditions.

Description

1328~97 039.264 ENGINE-SENSING DRAFT CONTROL SYSTEM
WITH MULTIPLE FEEDBACK COMPENSATION MECHANISMS

BACKGROUND OF THE INVENTION~

Field of the Invention The present invention relates in genersl to automatic feedback control systems for implement positioning means ;D off-road a8rieultural aDd oonstruction industry vehicles, and iD particular to automatic control systems for use on tractors for controlling the elevation of an implement attached thereto by connecting means such as a three-point hitch.

DescriDtion of the Prior Art Agricultura1 tractors have traditionally employed hydromechanical draft control systems wherein implemcnt pulling or load forces (i.e., draft) are scnsed throu~zh either meehanical or eleetronie sensor means eonneeted to or through an appropriate liDlcage. Mechanical linkages have inherently limited fle~ibility and high hysteresis. Electronie eoDtrol sensors sueh 8S load cells also are quite e~pensive and subject to damage due to o-~erloads, environmental e~posure and other problems.

Many different sehemes and systems have been developed to automatically control the elevation of an implemcnt attacbed to an adjustable hitch on a tractor. U.S. patents which teach coDtrolling the elevation of an implement or hitch as a - function of measured draft include:

Patcnt No. Inventor 2,629,306 Rusconi 4,300,638 Katayama et al.
4,301,870 Carrc et al.
4,343,365 Rajagopal et al.
4,437,048~ Arnold 4,508,176^ Wiegardt et al. ~;~
4,S18,044- Wiegardt et al.

1328497 ~.
039.264 Other U.S. patents disclose schemes aDd systems for controlling hitch position as a function of wheel slip. A determination of slip is based on a comparison of the speed of one of the driven wheels of the tractor with the true ground speed, as determined by monitoring the speed of an undriven front wheel or by radar means. Such patents include:

Patent No. Inventor 3,834,481 Carlson 4,086,563 Bachman 4,344,499 Van der Lely et al.
4,419,654 Funk 4,485,471 Herwig 4,518,044 Wiegardt et al.

U.S. Patent No. 3.716,104 to Koenig et al. shows the concept of controlling hitch position as a function of torque on the tractor due to implement load or draft when compared to eDgine RP~ U.S. Patent No. 4,465,142 to Van der Lely et al.
discloses an alternative coDtrol system which compares the desired cngine speed, as set by a manual Icver, against actual engine specd to obtain a difference signal which is then amplified aDd used to directly control an clectrically acti~ated hydraulic valvc tlIat raises or lowers a plow.

Other systems have bceD developcd which monitor actua1 engine speed. For e~ample, U.S. Patent No. 4,077,47S to Hino et al. discloscs a hitch control system with a ~rotary~ draft control which monitors both actual cngine speed and actual hitch position. In this control modc, two inscnsitivity thrcshold sensors are used to create a deadband to prevent hunting and chattering of thc solenoid valvcs. The difference between actual engine speed and desired engine speed is compared against uppcr and lowcr threshold values and is used to iDflucncc the operation of the hitch posi~ionin~ control when the eni~ine speed is between predctermined upper and lower speeds. Furthermore, below a yet lo~r en~ine speed, thc hitch control systcm automatically raises the hitch to prevent the enginc from stalling. The purpose of this modc tD keep ths working depth Df tbe implement near its desired valDe, while 039.264 effcctively preventing any accidental en8ine îailure due to overload conditions caused by an en8ine load which is too heavy for the engine to handle. This patent, however, apparently fails to recognize that the amount of engine lug-down from a predetermined set point or no-load speed can be used as a form of draft load control, since it provides a separate mode, the draft mode, which makcs use of a draft force transducer to provide a constant load on the tractor by automatically adjusting the height of the hitch.

It has receDtly been recognized that con~pletely eliminating mechanical draft sensors and instead using engine speed to determine draft or load force would be bcneficial. In D. Rutkowski ~ J. Welchans, ~The Development of An Electronic Draft Control System at Ford Tractor Operations,~ Proceedings of the Nationat Conference on Fluid Powcr (held in Detroit, Michigan on April 29 - May 1, 1986), pp. 301-306, a microprocessor-based systcm where actual engioe RPM is compared with the e~pected no-load RPM at a given thro~tle position is disclosed. This difference, namely the RPM deviation from no-load RPM, is used to calculate draft force by transforming the differencc into a draft signal by matching it to the engine's performance, which is represented by a specific point on one of seversl engine torque curves stored in the system's memory. The system then uses this draft force signal to control a proportional electrohydraulic valve that raises or lowers thc hitch. This articlc shows that such a draft force signal may be used in combination with 8 bitch position feedback signals~ if desired.

In order to operate, then the system ~ust not only be programmed with the specific engine torque curvcs, but must also know which onc of the several torque curves to use, which requires knowing what 8ear the tractor's transmission is in.

Our work with engine-sensing draft coDtrol systems shows that the draft control system describcd in the aforemcnt;oned srticle has a number of limitations.
In particular, a typical tractor is uscd with a wide variety of implements, somc of which tend to dig themsclves into thc ground, and Dthcrs of which tend to drive themselves out of the ground. Morcover, the weight of the implcn~ents varies dramatically, and this advcrscly influcnccs the stability of the syste--. ~)ur e3~pcriments show that a number 1328~97 039.264 of additional feedback eompensation mechaoisms, iocludiog manual controls for allowiog the operator to make field adjustments, are oecessary or highly desirable if an engine-seosing draft cootrol system is to be effective for a wide va~iety of implements.
Io particular, an engine-sensing draft eontrol system whieh only allows the operator to adjust the desired load teods to e~hibit valve chatter, huoting and iostability in a number of situations. Moreover, if the wheels begin to slip, which allows engine RPM
to increase, the system mistakenly perceives that draft load is tessening and responds by lowering the hitch, which can resu1t in the implemeot becomiog dug in, thereby stalling the tractor. The system deseribed in the aforementioned artiele requires the storage of families of engine torque eurves, either as tab1es of values (which ean take ioordioate amounts of memory in a microprocessor-based eontrol system~ or as eomple~ formulas (which ean be difficult to program into a miero-processor-based controller and can require significant computation time to eonvert an engioe lug-down value to a draft load foree using such formulas). Another problem with using pre-programmed eogine torque curve tables or formulas is that they are inaccurate in proportion to the variation of engioe performaoee from ideal eooditions. As the engine wears, %oes out of tune, or is misadjusted, the accuracy of the conversion from engine lu~-down to draft load using such torque curves beeomes inereasingly inaccurate. Also, knowledge of the gear ratios of each transmission aod the actual 8ear the tractor's transmission is operating in must be obtained, whieh adds to the eost and eomple~ity of implementing the system.

In light of the foregoing problems, it is an object of the present invention to provide an engioe-sensiog draft eontrol system whieh permits the operator to maoually adjust a sufficieot number of key cootrol system parameters to allow proper operatioo of the control system with a wide variety of implemeots of differeot weights and grouod-engagement characteristics. A further object of the invent;on to provide a cootrol system which is stable in operation and can eompcosate for the several nonlinear;ties commooly assoriated with traetor en~ioe throttles and the hydraulic valve used to operate a three-point hitch, including those introduced by implements of varying weights. Another object is ~o provide a draft control system which monitors slippage and can avoid e~ccessive slippa8e. Yet another object of the present invention is to provide nn engine-sensin~ draft control system ~vhich does not require knowlcdge of the 1328~97 39.264 cngine torque curve characteristics of the vehiclc or knowledge of the particular 8ear in which the vehicle is operating in order to operate effcctively.

SUMMARY OF THE INVENTION

In light Or the foregoing problems and objects, therc is provided an automatic feedback control system for a self-propellcd vehicle. Thc vehicle has an engine, ground-engaging traction means, such as wheels or continuous treads, for moving the vehicle relativc to the ground, and connectin8 means for attaching a 8round-pcnetrating imp1ement to the tractor. Actuating means, such as an electrohydraulic valve and cylinder, are provided for adjusting the elevation of the implement, or at least a first portion thereof attached to thc connection means, so as to vary the ground penetration of the implement in responsc to a control signal applied to thc actuating means.

In a first embodimcnt of the invention, the automatic feedback control systcm comprises: first sensing mcans for ~enerating a speed parameter signal rcpresentative of the actual speed of the engine; first input means for producing a desired no-load cngine speed signal corresponding to a desired no-load engine speed;
second input means for producing a tar8et speed signal which corresponds to ~ desired tar8et engine speed which is less than the no-load enginc speed; first difference means for gencratin~ a speed error ~ignal reprcsenting the difference between the speed parameter signa~ and the tar8et speed signal; and first control mcans for operating thc actuating mcans to ~djust the elevation of thc first portion of the implcment at least partially in response to the speed error signal.

In a preferrcd cmbodiment, the automatic feedback control system is implemented using a microprocessor-based elcctronic control system. The self-propelled vehicle may be an agricultural tractor, and the ~round-engaging traction mcans may bc at least a pair of driv n whcels. The connectin~ mcans may be a convcntional threc-point hitch assembly which may ;DCIUde a pair of drawbar links or draft arms which are pivotal~y snpported in spaced rel3tion at their for~ard ends on thc 039.264 1 32 84 ~ 7 tractor. The arms are raised or lowered about their forward pivotal a~es by the actuating means, which typically is a hydraulie lift assembly inclllding a pair Or spaced cylinders.

These and other aspects, objects and advantages of the present invention will be more fully understood from the follovving detailed description and appended claims, taken in conjunction with the drawings.

- BRIEF DESC~RIPTION OF THE DRAWIN~5~

In the accompanying drawings, where identical referense numesals or reference characters represent like items shown in the various figures:

Figure 1 is a combination diagram which shows the automatic fçedback control system of the present invention including an opcrator's console, an electronic controller in block diagram form, and a simpli~ied side ele~ational ~iew of a tractor having a three-point hitch assembly and towiDg an implement, and which also shows the various interconnections between the console, controller and various component~ and sensors on the traetor;

Figure 2 is a simplified control diagram of the control system of Figure 1 showing its major functional sub-systems;

Figure 3 is a detailed block diagram showing the signal flow and signal processing employed in the control system of Figure 1;

Figure 4 is a pair of signal ma~nitude vcrsus time graphs having a common time a~is and showing typical smoothing or time-a~era~ing eharacteristics oî
the delay modules of Figure 3;

Figure S is a graph illustratlng the technique of the present invention for ca1ibrating throttle lever posi~ion to no-load engine speed;

03g.264 Figure 6 is a graph of the transfer function of a typical deadband module illustratiDg its adjustment by the sensitivity pot;

Figure 7 is a graph illustrating the transfer function of the gain block in the engine speed feedback loop of Figure 3;

Figure 8 is a graph illustrating the traDsfer functions of the 8ain blocks in the engine acceleration feedback loops of Figure 3;

Figure 9 is a graph showing the transfer function block ;D of the slip sensing subsystem of Figure 3;

Figure 10 is a graph of the transfer function of the ma~imum lower rate block of Figure 3;

Figures IIA and IIB are graphs showing the input volta~e versus percent flow characteristics of an electrohydraulic valve used in the three-point hitch shown in Figure 1; and Figure 12 is a graph of the transfer function of the valve compensation block of Figure 3.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT~ ¦

Referring now to Figure 1, a four-~heel drive articulated tractor 30 includes an internal combustion engine 32, sueh as a diesel engine controlled by a speed control assembly 34 includin~ a throttle lever 35, a front pair 36 of conventional driven wheels and a rear pair of driven wheels 38, and a solenoid-operated powershiî t transmission 33 with an electronic transmission control1es 31. Transmission 33 is connected to thc engine 32 by suitable connecting means 17 and to the rear wheels 18 by anotl-er suitable conDecting means (not shown). A rear housing 20 on the tractor 32 039.264 supports a rear a~le 21 and a rock shaft 44. An implemcnt hitch 46, such as a conventional three-point hitch, is mounted to a rear frame 42 of rear housing 40. Hi~ch 46 includes draft links 48 which are connected to 1ift arms S0 via a pair of lift links S2.
The lift arms 50 may be ri~idly connected to the rock shaft 44 to ensure simultaneous and equal movement. The lift arms 50 are raised and lowered via a pair of parallel connected hydraulic lift or rock shaît cylinders 54. A drawbar (not shown) wouldtypica11y extend rearwardly from a frame S6 of thc three-point hitch assembly 46mounted on frame 42. The tractor 30 and the hitch 46 are merely e~emplary, and those skilled in the art will understand that the invention can be applied to tractors and hitches of other configurations. For e~ample, this invention can be used on two-wheel drive tractors or front-wheel row-crop tractors.

Ground-engaging implements, such as plows, discs, cultivators and rotary hoes, may be interchangeably mounted on or attachcd in a conventional manner to the draft links 48. In Figure 1, a moldboard plow 57 is shown connected at thc rearward end ~8 of the draft links 48. The communication of hydraulic fluid to and from the cylinders 54 through a suitable hose S5 and tubing or coupling means 59 is controlled by a conventional solenoid-operated electrohydrau1ic directional control valve (EHV) 62.
Valve 62 receives an electrica1 control signal VCS proYided on signal path 61 from contro11er 60 which is the brain of automatic feedback control system of the present invention illustrated in Figure 1. Controller 60 is preferably a microprocessor-based electronic con~roller, as will be described. ~alve 62 may bc comprised of a commercially aYailable servo-valve with a torquc motor- opcratcd pilot stagc and an integral second stage. Under controlled pressure conditions, the output hydraulic flow rate is substantia11y proportional to the magnitude and polarity of the electrical current control signal ~CS applied to the torque motor of thc valve 62. One such prcferred vslve is the Z4900 Serics electro-hydraulic propor~ior al three-way valve, availablc from Dyne~/Rivett, Inc. of Pewaukce, Wisconsin.

039.264 Located at any suitable position within the cab 68 of tractor 30 is an operator's console 70 which contains ~rarious input devices used with the control system of the present invcntion. One possible configuration of operator's console 70 is shown in an cnlarged repreSCDtatiOn within dottcd lines at the top of Figure 1. The opcrator's consolc 70 includcs the throttlc lever assembly 34, a display pancl 72 which may includc a visual display 74 snd a cluster 76 of indicator lights 78-86, two momentary contact (SPST) pushbuttons 88 and 90, and five potentiometers 95-100, all connccted in conventional fashion to electronic cootroller 60 by suitablc conductors represented by signal paths 102-120, as shown. The display 74 n~ay be of thc alpha-aumcric or numeric-only LED or LCD typc such as the four digit seven-segmeot displsy shown, and may be used to display appropriate messages re8ardin8 the status of the control system, in~luding desired and actual parametcr values as monitored by the control system 60, if desired. Thc data or signal path 10~ typically will include multiple conductors ss necessary to drive the display in conventional fashion. The light clustcr 76 may similarly bc used to display error codes and other indications of the status of the control system 60.

The throttle lever assembly 34 may include a lever position transducçr 124 (such ss 8 rotary potentiometer connected at the pivot point of Iever 33, as show~ in phantom). Pot 124 generatcs a speed command signal which represents the desiro~ no-load engine speed signal (ESD) that is provided ovcr linc 102 to controller 60. As illustrated by dashcd line 126, the thrott1e lever 3~ is connected by suitablc mechanical or othcr linkages to the throttle 128 of engine 32.

The five potentiometers 92-100 are all adjustable by the operator and may be of any conventional style and are prcferably linear rotasy potentiometers with calibrated markings thereon for easy and accurate visual readin~ of their se*ings by the operator. ~Hitch position~ pot 92 is used to specify the desired hitch position (i.e., - elcvation) between a minimum va1ue in the e~tremc counterclockwise (full CCW) or ~I)N" position and a ma~imum valuc in the e~tremc clockwisc (full CW) or ~UP" position.
Thc output of pot 92 is an analog desired hitch position signal HPD on liDe 112. ~Load~
pot 94 allows the operator to specify the appro~imate draft force hc wishes the control ,9 039.264 system to attempt to maintain. The setting of this pot varies from a minimum value in the full CCW or ~LO~ position to a ma~imum value in the full CW or ~HI" position, and produces an analog load signal LD on line 114. The setting of pot 94 also controls the amount of mix between the hitch position control subsystem and the draft controlsubsystem of controller 60, as will be further e~plained.

"Sensitivity~ pot 96 sllows the operator to adjust the sensitivity of the control system between a minimum value (in the ~MIN~ or full CCW position) to a ma~imum value (in the ~MAX~ or full CW position), and produces an analog sensitivity signal ST on linc 116. As will be e~plained, the sensitivity pot is uscd in thc prcfcrrcd cmbodiment to adjust the amount of deadband in the various fccdback loops of the control system of the present inYentioD, so as to avoid unnecessary hunting and valve chatter.

~ Lower Rate~ pot 98 allows the operator to adjust the maximum rate at which the hitch will be allowed to lower. The slowest rate of lowering is achie~ed when pot 98 is in the ~S" or full CCW position, and the fastest ratc of 10wering is allowed wheD
the pot is in the ~F~ or full CW position. The output of pot 98 is the ana10g lower rate (LR) signal LR on line 118. 'Slip Rate~ pot 100 is used by the operator to adjust the minimum poin~ 8t which ~he sys~em begins to take action in order to correct for e~cessive slippage between the driven ~vheels of tractor 30 and the ground. The minimum slip rate is achieved when pot 100 is in its ~MIN~ or full CCW position, while the ma~cimum slip rate which is achieved by placing pot 100 in its ~MAX~ or full CW position.

Input devices 130 throush 140 may be momentary contact (SPST) pushbuttons, selector switches or the 1ike. The pushbuttons 130-140 each have thcir own signal path, such as conductor 128, leading to the controller 60. Pushbutton 130 is a lainp test button. Each of the other pushbuttons 132-140, when depressed, causes the value of the parameter indicated by its respectivc one of the poten~iometcrs 92-100 directly therebelow to be displayed on the dlisplay 74, so that the operator can easily ~el1 thc precise settin~ of each pot and verify eorrect operation of the pDrtiOn of the control system used to read these pots. Pushbuttons 130-140 are optional.

132~497 039.264 The tractor 30 also includes a conventional hydraulic power supply 144 connected in conven~ional fasl2ion to the powcr plant 32 and/or transmission 33 to provide the necessary pressurized hydraulic fluid to operate the valve 6~ and cylinders 54. In a prototype of thc present invcntion, the rods 146 of siogle-acting cylindcrs S4 arc raiscd or e~ctended by the app1ication of pressurized hydraulic fluid dirccted by valvc 62 from thc powcr supply 144 throu~h hoses 55. Rods 146 of cylinders 54 are lowercd by the forcc of gravity bcaring upon the front portion 148 of thc implemcnt and movablc arms and links of the hitch 46 and c~certing a downward force upon the rods.
which, whcn thc valvc 62 connccts hoscs 55 to thc tank of thc powcr supply 144, causes hydraulic fluid to bc mctercd the valvc ~2. By regulating thc size of the opening to tank, valve 62 controls the rate at which the rods 146 are lowered. Those in thc art will appreciatc that the clectrohydraulic valvc 62 could be readily arrangcd to operate double-acting hydraulic cylinders whcre prcssurizcd hydraulic oil is supplied to the rod cnds as well as the cap ends thcreof, thcrcby using hydraulic forcc to lowcr as well as to raise the rods of the cylindcrs.

The tractor 30 includcs scvcral transduccrs uscd to monitor kcy parametcrs or conditions of thc tractor during operation. Spcciî ically, an engine speed transduccr 150, which may bc a conventional variable reluctancc magnetic pick-up that senscs thc movcmcnt of thc tecth of the rotating enginc flywhecl, is uscd to gencratc an actual cnginc spced s~gnal ESA providcd on signal path 152 to the controller 60. In a similar fashion, speed transducer 154 is uscd to dctect the rotation of rear a~lc 41 as the rcar whecls 38 rotate thcreabout, thus providing an indicatcd ground speed (GSI) signal on linc 156 to controllcr 60. The true ground spccd may bc determincd by a conventional Dopplcr radar horn 160 shown mountcd at thc lower front end 162 of tractor 30 by bouncing radar signalsoff of tlse ~round 161. Thc devi~e 160 providesan actual ground spced signal GSA indicating the true ground speed to thc controller 60 ovcr signal path 164. This signal may take the form an analog signal or a digital signal whosc fre~ucncy or period varics in accordance with the truc sround speed of tractor 30.
Thc indicatcd ground spced and actual ground spced signals GSI and GSA are uscd to determine the lip rate o- the tractor durin8 ooer-tion. Aoy other suit-ble technique 039.264 may be used to determine the actual or indicated ground speeds. For e~amp1e, if front wheels 36 wcre not driven, but instead rolled freely alon~ the ground, a transducer like transducer IS4 could be mounted to scnse the rotation Or the front a~le 166 carrying wheels 36.

The microprocessor-based controller 60 is of conventional dcsign and includes a microprocessor 170, onc or more digital l/O ports 172, one or moreanalog-to-digital converters 174, an erasable, programmable read-only memory (EPROM) 175, an elcctrically erasable, programmable read-only memory (EEPROM) 176 for storing data to bc rctaincd during power-down situations, programmable timer modules 177 and 17B, and a convcntional modulation (PWM) generation circuit for producing a PWM signal ~ICS sent via path 61 to the clectrohydraulic valve 62. The modules or blocks 170-178 are interconnected by a suitable bus 184 having multiple connectors for simultaDeously carrying data, addrcss and control signal information between the blocks, in 8 maoner well understood by those in the art. In a preferred embodiment of the controller 60, the components used for these various blocks are listcd in Tablc I below.

TABLE I
REF. NO. ITEM ~A~UFACTURER PART NO.

170 Microprocessor Motorola MC6802 171 Static RAM Motorola MCM6810 172 Digital I/O Port Motorola MC6821 175 EPROM Intel 27128A
177, 178 P.T. Modules Motorola MC6840 179 EEPROM National Semiconductor NM9306 Tbose in the art will readi1y appreciatc that other microprocessors and electronic configurations for implemcnting the control system 60 are also practical.

i The pu1se-width modulatioD generation ci rcuit 180 produces a iuitable positive or negative currcnt signal VCS of adjustable duty cyclc to the electrohydraulic valve 62 ovcr a pair of wires reprcsented by signal path 61. The circuit 180 has an inputs threc signals, the first two of which originate over lines 188 and 190 fromdigi~al I/O port 172 and are binary si8nals. Thc first of thcse binary signals indicates whcther tbc circuit ItO is to be on or o~t, nd the second n- thc binary silnals indicates wbcther 1328497 3~._o4 thc circuit is to producc a positivc current (for raising thc implement) or a negative current signal (for lowering the implement). Circuit 180 receives a third signal over`
line 194 from the second programmable timer module 178. Module 178 is programmedby instructions scnt from the microprocessor 170 and produccs on line 194 a di~ital signal havin8 a rclativcly high frequency which has an average DC value proportional to the spccific duty cycle dcsired for thc PWM signal VCS spplicd to lincs 61 ;D a wcll-known manncr, which necd not be dcscribed here. Further detai~s of thc implementation of a similar electronic controllcr using likc components is providcd in commonly a~signed copending U.S. patent application Serial No. 055,820, filed May 29, 1987 in thc namcs of K.L. Brckkestran and J.C. Thomas and cntitled "Electronic Control Systcm For Power-Shift Transmission,~ U.S. Patent 4, ~55, 913 gMnted on August 8, 1989.
_ . .
Figure 2 is a gc~eral block dia8ram showing thc functional subsystems and modules or blocks of the contro1 system of the present invcntion that are found within thc e1ectronic contro11er 60, and which may be referred to co11ectively as coDtrol syst,m 200. The control systcm 200 has four mapr functional subsystcms which are represcntcd by the position fccdback control module 202, thc enginc-sensing draft feedback contro1 module 204, the upshift/downshift control modu1e 206 and the s1ip-scnsing contro1 module 208. Other significant functiona1 blocks within thc system inc1ude the ma~cimum 1Ower rate contro1 modu1e 210, the e1ectrohydrau1ic valve compensation modulc 212, thc learn mode control module 214 used to pro~ram or sct up the compensation modu1e 212, the throttle compensation modulc 216, and the calibration mode control module 218 used to program or set up the throttlc compensation module 216. Blocks 202-218 each makc an important contribution to increasin~, thc overall effcctivencss of thc automatic fcedback control system of the present invention. The engiDe-sensing draft control 204, the upshift/downshift control 206, thc slip-sensing control 208, the ma~imum lowcr rate control 210, the combination of the EHV
compensation modulc 212, learn mode control 214, and the combination of thc thsottlc compcnsation modulc 216 and the calibration modc control 218 each arc belicvcd to represcnt a significaDt contribution in thcmsclves to thc advancement of the electronic draft control art. As those in the art wil1 readily &ppreciatc, it is not nccessary to usc 1328~97 039.264 all of the modules or blocks 202-218 to have an effective draît control system. For e~ample, the engine-sensing draft contro1 204 snd slip sensing control 208 could be used without the position control 202. Similarly, the engine-sensing draft control 204 can be used with the ma~cimum rate lower control 210 and the compensation modules 212 and 214 without using the upshift/downshift control 206 or the slip-sensing control 208.
Those in the art will readily see other combiDations of thc major blocks of the present invention which arc capable of operating independently as an effective automaticfeedback draft control system. However, it will also be apprecisted that the overall performance of the draft control system of the present invention improves as more and more of these blocks and modules are used in combina~ion.

The overall control system 200 also includcs a position and draft control signal m;~cer block 220 which receivcs as inputs a position crror signal PES over signal path 222 from position control module 202 and a draft error signal DES over signal path 224 from the engine-sensing draft control module 204. The mi~cing module 220 allows the operator to manually select, by adjusting the load pot ~4, the relative f influence that the error si~nals PES and DES from modules 202 and 204 will have upon the raising and lowering of the hitch. Specifically, as the load signal LD on data path 206 increases in value, the relative influence of the signal PES is decrcased, while the relativc influence of the signal DES is increased, in a manner that will be madc clear shortly.
i The position control 202 forms the portion of thc position error loop in the control system of the present invention. It receives as inputs the desired hitch position si8nal HPD, the actual hitch position signal HPA and the sensitivity si~nal ST, which is used to adjust the size of the deadband in the position error feedback loop.
The en8ine-sensing draft control 204 forms the main portion of the draft error feedback 1P ;D the control system of the present invention. It rece;vcs as inputs thc sensitivity signal ST which is used to adjust the size of various deadbands in the draft error fccdback loop, the dcsircd engine speed signal lESD, as compensated by module 216, and the actual cnginc specd signal ~SA. ~he draft control system 204 is part of a closcd loop control algorithr~ implemtnted in lhe electronic controllcr 60 ~vbicb com~ares ~he 39.264 desired engine speed sgainst the actual en8ine speed and sends the draft error signal DES
through mi~er 220, lower rate control module 210, signal summer 226, and value compensation module 212 to the valve 62 to raise and lower the implement so as to maintain the desired value of en8ine lug-down specified by the si~nal LD from the load pot 94. The draft control system of the present invention relies upon the fact that the amount of engine lug-down, that is, reduction in engine speed or RPM from a desired no-load en8ine speed or RPM specified by si8nal ESD from the throttle pot 12~, generally corresponds to a speeific load force being 8enerated by the en~ine 32 as it pulls the implement 57 through the ground. Assuming that tractor 30 is being operated uponreasonably level or gently sloping ground, where the load placed upon the engine due to the work which must be done to move the tractor over the ground is relatively constant, the variations in engine load will correspond to variations in draft force caused by conditions associated only with the implement, and the variations in engine speed will correspond to variations in draft load. We recognize that there can be a fair amount of uncertainty as to precisely what the actual draft (as measured in pounds or Dewton-meters) is upon the tractor when measured by the amount of engine lug-down. This is particularly true when the 8ear in which the vehicle is operating the engine torque curve, and the amount of engine power dissipated as drive train losses and in the dynamic and continuous deformation of the driven wheels, are not known ~y the electronic controller 60. However, we have discovered that this uncertainty as to the precise draft being produced by the implement is not an impediment to the farmer who simply wishes to operate his tractor/bitch/implement combination at a relativelyconstant draft perceived as optimum by him without having to eontinuously attend to the adjustment of the elevation of the hitch to prevent stall outs, di8 ins and to avoid unnecessarily shallow plowing, cultivation or the like. Therefore, of far greater importance than a know1edge of the precise draft which the implement S7 is ge~erating is the ability of the control system 200 to ensure that (I)tractor 30does notstall out,(2) implement 57 does not become du8 in and (3) the hitch position is at or near its lowest - practical elevation without constant hunting and other instabilities associated with some other p}ior art draft control systems.

039.264 The upshift/downshift cootrol 206 receivcs as inputs the sensitivity signal ST, the compensated desircd en8ine speed signal ESD' and the actual en8ine speed signal ESA. When the control system 206 detects that the actual enginc speed has fallen too far below the tar8et point cngine speed, it generates a downshift signal DNS which is delivered via signal line 228 to the transmission controller 31 associatcd with and regulating the operation of transmission 33, which causes the transmission to bc shifted into a lower gcar, thercby providing morc torque to prcvcnt cngine stall-out. When the engine speed increases to a prcdetermined levcl, such as within a predctermined number of RPMs of the dcsired no-load en8ine speed as indicated by signal ESD', the control systcm 206 will produce an upshift signal UPS which is delivered via line 232 to transmission controller 31 and which causes thc transmission 33 to be shifted to the ne~t higher 8ear. The predctermined upshift threshold speed may be set at any suitable value above or below the desired no-load cngine speed. The upshift signal mcans and the downshift sigDal means cach may also and preferably do monitor a rate change of engine spced. Thus, the downshift si8nal would only be produced after the actual engine speed has fallcn below a minimum downshift îhreshold specd and the eDgine speed is decreasing at at least a predetcrmined minimum ratc, which suggests that the engine will definitely not be able to recover without downshifting the transmission 33.

The throtele compcnsation r~odulc 216 and calibration mode control module 218 ar~ uscd to compensate for the nonlinearitics 'between thc position of throttlc lever 35 (as the input) and the correspoDding no-load cngiDe specd produced by such settings. Tbe calibration mode control 218 causes thc clectronic controller 60 to perform a scquencc of steps in accordance with programmed instructions resident in the static RAM 171 to allow the nonlinear characteristics to be determined while the operator moves the throttle lever 35 in a spccified manner, as will be furthcr explaincd with rcgard to ~igurc 5. In a similar manncr, the valve compensation module 212 ~nd the learn mode control 214 allow the electronic controllcr 60 to adjust for thc nonlinear electrohydraulic charactcristics of ihe valvc 62, which are heavily influenced by the weight of the implcment 57 upon the draft arms 4~, which weiRht can dircctly and proportionally affect thc force and thcrefone thc hydraulic pressure a~d flow during the raising and lowering of hitch snd implement.

s 132~497 039.264 Figure 3 is a detailed block diagram illustratin~ the signal flow and inner workings of the feedback loops and control algorithms of control system 200 implemented in electronic controller 60. Those skilled in the art will readily recognize and be able to interpret the various graphical symbols and blocks shown in Figure 3 and will also apprcciatc that the automatic feedback control system 200 shown in Figure 3 may be implcmcnted by using conventional d;screte elcctronic circuitry and hardware, or by using a programmcd microprocessor-based controller with integratcd circuit components of the typc rcferred to in the description of Figurc 1. Armcd with the detailed description herein, those skilled in thc art will be readily able to implemcnt thc control algorithms and strategies described herein, using routinc electrical circuit design and/or programming skills, without undue cl~pcrimentation. To the c~tcnt that equations would bc hclpful or arc necessary to the understanding of the invention, they arc describcd below, shown in the signal flow rcprcsentation prescnted in Figure 3, or in the graphs of various functions in Figures 4-12.

The various modules shown in Figure 2 are prcsented once again in Figure 3 in more detailed form. Spccifically, the modules 202-208 are shown as blocks formed by dashed lines, with one or more control blocks therein. The control system 200 shown in Figure 3 includcs input blocks 240-26~, shown along the far lefthand side of Figure 3, which represent coDvcntional signal-conditioning circuits and/or buffers uscd to convcrt the unfiltered digital and analog input si8nals shown in Figure I into usable di~ital sigDals of the type which can be processed by microprocessor-bascd controller 60. Analog input signals such as the signals from thc potcntiomcters 64, 92-100 and 124 shown in Figure I require conversion to correspondin8 digital values. This function is performed by analo8-to-digital convertcr 174 shown in Figure 1 and is functionally represcnted by respectivc input blocks 244,242, 248,246, 240 and 252 shown in Figurc 3. Thc corresponding digital value for the actual hitch position si~nal HPA is indicatcd by an asterisk following the symbol HPA on line 264. Similarly, the digital reprcsentation of the learn signal LN on linc 108, transformed by input block 250, is shown as signal LN~ on linc 2~6. The actoal engine speed si8nal ESA on line 152, the indicated ground spced signal GSI on line 156, and the actual ground speed signal ~;SA
_17_ 039.264 on linc 164 preferably are pulsating signals which each have a frequcncy proportional to its sensed parameter. The determination of the avsrage period er frequency of these signals is performed by programmable timer module 177 ~hown in Figure 1. Upon request from microprocessor 170, module 177 outputs digital values onto bus 184 which cosrespond to the respective sensed paramctess, namely actual engine speed, indicated ground speed and actual ground speed. This measurement and transformation function is represented by input blocks 254, 260 and 262.

In a linear control systcm of the type shown in Figure 3, a rapid change in a command si8nal~ such as desired hitch position HPD, the sensitivity signal ST, the load signal LD, or the slip ratc signal SR, can produce abrupt and undesirable rapid movemcnt of the implement. To greatly reduce or elin~inate the opportunity for the operator to introduce such transient conditions into the eontrol system 200 by abruptly changing one or more these signals, an averaging or delay function is emp1Oyed to smooth out rapid changes in such digital signals. This delay function is represented by delay blocks 264 through 272 which respectively further condition the digital representation of signals HPD, ST, LD, ESD snd SR. A typical e~ample of this smoothing or filtering function performed by blocks 264 through 272 is illustrated in the time graphs of Figure 4. Scgmented line 274 in the uppcr graph represents the average value of the digitized output of an e~emplary pot, such as pot 92, which is changed from a minimum value at point 276 to a ma~imum va1ue at point 277 in about 0.2 seconds.
The delay block, such as delay block 264 shown in Fi~ure 3, produces an output on signal path 278 as shown by segmented line 280 in the lower graph of Figure 4. Line 280 illustrates that the output signal on path 278 rises from the same minimum va1ue at point 282 to a ma~imum value at point 284 in a considerably longer period of time such as 0.9 secoDds. Similarly, when the digitized value of line 274 begins to fall at point 286 from its ma~imum value to its min;mun~ value al point 288, thc output signal on path 278 falls much more slowly as shown by the sha}lower slope betwcen points 290 and 292. Those in the art will readily appreciate that there are similar convcntional averaging techniqucs for performing sucb smoothing or filtering function which are all well-suited to prevent undesired transient conditions being induced by rapid adjustment of the command pots.

1~2~97 039.264 The conditioned and filtered value of the desired engine speed signal output on line 298 is further conditioned by transfer function block 300 in order to compensate for the nonlinearity between the relative position of the throttle lever 35 as determined by rotary pot 124 in comparison to the no-load engine speed which is produced by such setting of lever 35. Figure 5 is a graphical representation of this nonlinearity. In the preferred embodiment Or the present invention, rotary pot 124 has linear output characteristics so that the degree of pot rotation eorresponds to the an8ular movement of the throttle lever 35. Howe~er, due to the ;nherent nature of the conveDtional linkage (represented by line 126) between the throttle Iever 35 and the throttle 128, as well as the nonlinear characteristics between the position of throttle 12~
and the actual no-load engine RPM, the relationship between throttle position as determined by the pot 124 and the engine speed can be and typically is rather nonlinear.
Dashed line 304 in Figure 5 represents an ideal linear relationship while line 306 represents a typical actual characteristic between throttle pot position and no-load engine speed.

Calibration mode module 218 is provided to simplify the aequisition of the data required to calibrate or set u~ transfer function block 300 for a given tractortengine combination. Module 218 represents electronic coDtroller 60 bein~
programmed to select three or more points along curve 306 where data is acquired, so that a linear interpolation between the two adjacent points nearest the actual throttle pot setting can be performed by block 300 to determine or closely appro~imate the true DO-load engiQe speed that is desired. The sequence of steps performed by the operator and the controller 60 to perform this calibration of transfer function block 300 will now be described. With engine 32 warmed up, the vehicle 30 ~t rest and transmission 33 in neutral, the operator advances throltle lever 35 from its minimum or ~LO" position until the first of the plura1ity of points, namely point 31û, is reached. The operator knows when he hasrcached point310 becausethe controller60 turnson thefirst indicatorlight 78 on the display panel 72. After a predetermined period of time, such 8S two or three seconds, to allow the en8ine speed to stabili2c, electronic controller ~0 takes a readin8 of the actual en8ine speed via signal E~A on liDe 152 and stores tbe read Yaluc 1328~97 039.264 corresponding to this RPM in its memory. Thereafter, the operator slowly advances the throttle lever until the second point 312 is reached, at which time the second indicator light 80 comes on to notify the operator that that point has been reached. After a delay of two or three seconds, a second reading Or the engine RPM is taken and stored. This procedure is repeated for points 314 and 316, with third and fourth indicator lights 80 and 82 respectively being turned on by the controller 60 when the operator has moved the throttle Iever 35 sufficiently to reach each respective point so that no-load eDgine speed readings can be taken as bcfore. In this manner, the controller 60 Iearns a sufficient numbcr of points along curve 306 in order to perform a reasonably accurate linear interpo1ation between points, as represented by straight linc se8ments 318, 320 and 322. Those skilled in the art will appreciate that a 8reater number of points could be selected if desired. We have found that in the control system of the present invention, thc calibration of the signal from the throttle pot 124 to take into account thenonlinearities of the throttle 128 and engine 30 bcfore it is used in thc feedback loops is e~tremcly important. It should bc appreciated that the graph of Fi~ure S also represents the transfer function performed by block 300. In other words, the valuc of the input signal on line 278 may be graphed along the horizontal a~is of Figure 5, and thecorresponding output signal may bc read upon the vertical a~cis of Figure S, along the segmented lines 318-322. ID practice. the controller 60 interpolates as necessary between the points 310-316 to calculate an output from any gi~en input. ID Figure 3, thehe~agonal block 326 and the data path 328 provided as an input to transfer function block 300 represent this sequence of steps required for calibrating block 216.

The position control 202 and the draft control 204 both utilize a second order feedback loop, which means that the derivative of thc first order feedback signal is taken. In the case of position control 202, thc first order fecdback signal is the actual hitch position HPA. In the draft control 204, the first order feedback si8nal is the actual engine speed ESA. Oval blocks 330 and 332 rcspectively reprcsent the function of taking the derivatives of the HPA signal on liDc 265 and the E~A signal on linc 334. Thcre arc scvcral wcll-known, digital techniqucs and several well-known analog techniques for takin8 dcrivatives of signals and elcctroni~ control systems which those skillcd in the art are famil;ar with and thus necd not be described here. The rate 039.264of change of actual hitch position is thus provided on signal path 336 while the rate Or change of actual eogine speed is provided on signal or data path 338.

Triangular blocks 350-374 are amplifiers which are provided to perform a scaling function upon respective signals passiDg therethrough. This scaling function is typically neeessary to place the 3ignals into a ran8e comparsble with the other signals with which they are to interact, and is well understood by those i~ the art.
Circular symbols 380 through 398 represent summation points where signals are combined. For e~ample, in position control 202, the signal output on data path or line 412 by amplifier 352 is subtracted from the signal output by amplifier 350 on line 410, and the resulting difference is output on line 414. The subtraction operation is indicated by the minus sign 416 adjacent to line 412. In mi~er block 220, the signal on path 418 is added to the signal from path 420 by summer 384, and the result is output on path 422. The other summers 382 and 386-398 operate in a similar manner to either summer 380 or summer 384.

In mi~er 220, the triangular bloeks 430 and 432 represent an adjustable amplification funetion. Amplifier 432 amplifies the draft error signal DES
on path 434 by the value k del;vered by data path 436, while the amplifier 430 amplifies the position error signal PES delivered on line 438 by the value (I - k) aad outputs the result on data path 418. The value of the parameter k is direetly proportional to the value of the load signal LD delivered by line 114 from the load pot 94. The amplifier 374 scales the signal on line 440 so that the value of k ranges between a minimum of 0.00 and a ma~cimum of 1.00. Thus, mi~ing means 220 provides a familiar technique for allowin~ a eontrol signal of adjustable magnitude to re~ulate the relative influence of two inpu~ signals, namely signals PES and DES in ~ccordance with the setting of the load pot 94. When signal LD from pot 94 is at its ma~imum, the draft error signal DES is provided to line 420 at fu}l strength, while the position error signal PES does not pass through amplifier 430. Conversely, when the load signal LD is at its minimum, only the posision error signal PES is passed throuph mi3cing block 220. Of course9 any desired decimal fraction combination of the signals PES and DES which adds up to 1.00 ca~ also be chieved by blocl: 220 by suitable adJustment the load pot 94.

039.264 1328~97 The trian8ular blocks 4S0 and 452 in the draft control 204 are drawn as open-loop differential amplificrs and represent signal comparators. Lines 454 and 456 shown connected to ~round represent input values of zero. When the signal on line 458 is abovc zero, the ou~put of comparator 450 is at a minimum va1uc or logical zero, while ~he output 462 is at a ma~cimum value or logical oDe. When the value on data path 458 is below zcro, the output 460 of comparator 450 is at a ma~cimum or logical one value, while the output 462 of comparator 4S2 is at a minimum or logjcal ~ero value.
The bow tie-shaped blocks 470 and 47-2 represent a function, which in an analog circuit might be called a gated switch. In other words, when the control signal on line 460 is in a logical one state, the data signal OD input linc 474 will pass through to output linc 476 with no change in value; when the signal on line 460 is in a logical zero statc, thc signal on input line 474 is not passed to line 476, and instead thç output valuc on line or data path 476 rcmains at zero. In a similar fashion, gated switch 472 passes the signal on input line 474 to the output line 478 when the 8ate line 462 is in a logical one state, while placing a zcro value on line 478 when the gate value 462 is in a logical zero state.

The transfer function blocks 480 througb 488 each have two inputs and one output, and may be called adjustable deadband blocks. Their collective purpose is to provide a deadband or dead zone about thc desired hitch position and about the target point engine speed so that the control system 200 will not attcmpt to make any adjustment to correct for hitch position error belov~ a certain absolute magnit~de, or to correct for enginc speed error below a specified absolute magnitude. Providing such dead zones about the desired hitch position helps ~reatly reduce value chatter and hunting, thus ~rolonging the life of the affectcd equipmeDt such as valve 6~ and hydraulic power supply 144. The input on the left of e2ch block 480-488 is a data signal, while the input on thc bottom from liDe 490 i5 an adjustable control signal, the value of which is directly proportional to the magnitude of the scnsitivity signal ST
from the sensitivity pot 96. Thc control sigoal ST' on line ~90 is used to vary the sizc of the deadband io the transfer function implemented by blocks 480 throu~h 488. A

typical transfer function for blocks 480-488 is shown in thc sraph of Figure 6. Each suc1 blDck providcs a dcadband resion about the vertical a~is of variable sizc depending -~2-`` 1328~97 39.264upon the setting of thc sensitivity pot 96. When the sensitivity pot 96 is set towards it minimum value, the size of the deadband indicated by regions 491 and 492 is large. The light, sloped lines 493 and 494 are strai8ht aod have a slope of 1.00, and indicate that any signal passing through one of thc blocks 480-488 is reduced in magnitudc by one-half of the overall size of the deadband of that block. As the sensitivity signal ST is increased towards its ma~imum value, the size of thc ovcrall central deadband providcd by the transfer function decreases, as indicated by the smaller deadband re~ions 495 and 496 and the heavy sloped straight lines 497 and 498. Lines 497 and 498 also have a slope of 1.00. Thus, cach of the blocks 480-488 provides a deadband zone which decreases the magnitude of the signal passing therethrou8h by an amount equal to one-half of its deadband value, as is inverscly determined by the value of the sensitivity signal ST.
The size of the deadband zoncs from block to block cao be different if so desired.

A number of other transfer function blocks are used in the control system 200 illustrated in Figure 3. The transfer function block 500 controls the gain of the speed fcedback circuit in the draft control 204, and its typical operation is illustrated by the nonlinear segmented or curved line 502 in Figure 7. Thc gsin of the speed fccdback csntrol loop is prcferably reduced in the vicinity of the vertical a~is as indicated by shallowly-slopcd line se8ment 504, which passes through the origin of the graph in Figure 7. The slope thcrcaftcr incrcases, as indicatcd by line segments 506 and 508, bcforc bcing prefcrably rcduccd somewhat, as rcpresentcd by shallowly-sloped line segments 510 and 512, at the outer limits of thc system opcration. Thc slopc of lines 506 and 508 may be made unequal if desired. Thc transfer functions of blocks 512 and 514 in draft control systcm 204 is illustrated in Figure 8 by straight lines 516 and 518 respectively, but can also be nonlinear if desired. Thcsc blocks control the gain in two paths of the acceleration fecdback loop with draft control m~dulc 204, as will be furthcr e~plained. Those in thc art will appreciate that thc nonlinear transfcr blocks shown in Figurc 3 can readily bc implemcntcd in software or firmwarc by using tables of programmed values, wherein the valuc of an input signal may servc as an inde~c to access the entries of a table, which correspond to outpue values.

039.264 ~ nother transfcr function block in cootrol systcm 200 is block 520 uscd in thc slip-sensing control systcm 208. The transfcr runction of block 520 will be c~cplained with rcspcct to Figurc 9. Transfcr f~nctioll blocks 530, 532, 534, and 536 will bc e~plaincd latcr with the e~planation of the upshift/downshift control system 206.
Transfer blocks 210 and 212, are used in the ma~imum lower rate control module and in the electrohydraulic valve compensation module, respectively, and will bc e~plaincd by illustrations in Figurc 10 and Figures 11 and 12. A divider function block 540 in thc s1ip-scnsing control systcm 208 simply dividcs thc input signal A by thc input signal B
and provjdes a resulting quotient as a signal on data path 542. In thc upshift/downshift control systcm 206, thc trapczoidally-shaped blocks 550 and 552 each rcpresent a single-shot function. Singlc-shot 550 is typical of both and it gcncratcs a momcntary logical one output on linc 554 in rcsponsc to a logical one input signal on line 5S3, and will not gencratc anothcr momentary positive pulsc until a reset signal is rcceived at its rcsct input connccted to line 556. Thc reset signal on line 556 is providcd by the output of the other single-shot block 552. Similarly, thc output signal of single-shot 550 is uscd to reset the single-shot 5S2.

The signal flow and gencral opcration of thc control system 200 may now bc dcscribcd. The hitch position feedback control 202 receives the desired hitch position signal HPD and the actual hitch position signal HPA from input buffcrs 242 ~nd 244, and thcn filters and scales the ~PD signal and scales the ~PA sig~al. Then, at a summing block 380, it subtracts the buffered and scaled value of thc actual hitch position on line 412 from thc buffered, fi1tercd and scalcd valuc of tbe desired hitch position si8nal on line 410. The resulting output signal from summer 380 on linc 414 is an amount below position (ABP) signal indicating thc rclativc amount or distancc that the hitch is below the dcsircd hitch position, indicatcd by tlhc scttiDg of hitch position pot g2. The deadband block 480 only permits an output signal to be provided on line 580 if thc magnitudc of the ABP signal e~ceeds onc-half Or the dcadband zonc size, as specificd by thc conditioned, filtered and scalcd value on line 490 representing the setting of the sensitivity pot 96. ~he hitch position rate (HPR~ signal on line 584 is similarly reduced in magnitude by deadband block 482 before being output on line 586.
The signal oo liDC 586 is tlb~--cted trom tbe i8oal OD linc ~BO by tulome- 3B2 to produce 39.264 the position crror signal on line 438. Thc PES signal is reduced in size by the multiplier value (I - k) where k is a vslue determined by thc setting of ~he load pot 94 as pre~iously e~plained. This reduced PES signal on line 418 is added to the signal (if any) on line 420, and the resulting signal is output on line 422, and after passing through transfer function block 210 is added to the signal from line S88 ~y summcr 386 to psoducc the combined control signa1 CCS on linc 590. The CCS signal is modificd by transfer function block 212 to producc thc final control signal F~S on linc 592 which is provided to thc PWM gcncration circuitry along signal path 592, which corresponds to conductors 188, 190 and 194 in the controllcr 60 shown in Figure 1. ThePWM gcneration circuitry 180 convcrts the FCS signal to the PWM signal VCS applied to the electrohydraulic valve 62.

Thc engine-sensing draft feedback control 204 forms part of a closed-loop coDtrol a1gorithm which regulates thc heiRht of the implcmcnt so as to maintain the engine speed at a calculated target point engine specd, which corresponds to a desired engine load sct by the operator using load pot 94. The tar8et point engine speed is calculated from settings of thc throttle Icver 3S and the load pot 94. The sctting of the throttlc Icvcr 35, as read by pot 124, produces the desircd cnginc speed signal ESD, which indicatcs the desired no-load enginc RPM (once the transfcr funct;on block 300 has been calibrated as described abovc). The conditioned, filtcrcd, calibrated and scalcd dcsired engine spccd signal may for convcnicnce be ~cfcrrcd to as signal ESD'. Thc conditioncd, filtercd and scalcd load signal LD' on line 440 is subtracted by summer 388 from the signal ESD' to gcneratc a target point signal TP, which corresponds to the desircd RPM that the tractor/cngiDc/implemcnt combination should ideally bc maintained at by the closed-loop draft control, if 100% draft control is sclected (that is, k ~ 1). Thus, the target point signal dircctly corresponds to a desired engine RPM undcr load. Thc conditioned and filtcred actual engine spced signal ESA' is subtractcd from thc targct point signal TP by summer 390 to obtain the aamount bclow targct' signal ABT
on line 600, which corresponds to the deviation of thc actual engine speed of cn~inc 32 below the tarBet point enginc spced. The ABT signal is rcduced in sizc by deadband block 484, which produces an output only i~ the value of the ABT signal e~ccecds onc-half of the size of its deadband. The gain block S00 then amplifics thc signal on line .~

039.264 602 in accordance with its transfer function, and this amplified speed crror signal SES
on line 604 is passed to summer 392.

The draft feedback control 204 may also 8enerate an amplifiedacceleration error signal AES on line 606 that is provided as an input to summer 392, where it is subtracted from the SES signal. The AES SigD~II on line 606 is produced in the following manner. The derivative of the actual engine speed signal from buffer 254 is taken by derivative block 332 and scaled by amplifier 366. The scaled output of amplifier 366 represents the acceleration of the engine and may be called the engine speed rate signal ESR, which is delivered by line 474 as an input to 8ated switches 470 and 472. If the signal ABT is positi~e, indicating that the actual engine speed is below the target point spced, comparator 452 turns switch 472 on, allowing the signal ESR to pass to line 478. If the signal ABT is negative, indicatiQg that the actual engine speed is above the tar8et point speed, comparator 450 turns on a switch 470, allowing the signal ESR to pass through to line 476. Switches 470 and 472 cannot be on at the same time.
Thus, it will be appreciated that transfer function blocks 486 and 512 condition the signal ESR only when the engine speed is above the tar8et point speed, while the transfer function blocks 488 and 514 condition the engine speed signal only wheD the engine speed i5 below the target point speed. Note that the gain imparted to the signal ESR by block 514 i5 shown as being 8reater than that of bloek S16. Through testing of prototypes of the present invention, we have determined that some additional ~ain is very desirable in the acceleration feedback circuit of draft control system 206 when tbe engine speed is below the target point speed, since it improves the performanee and response of the engine-sensing draft cont}ol system significantly. The amount of increased gain may be determined by e~cperiment for any giYen engine/val~e 62 combination. The summer 396 pro~ides the output of blocks S12 and 514 to line 606 without ehan8e. Thus, it will be appreciatcd that the second ordcr feedback signal ESR
is proYided to summer 369 either throu~h the seria1 path consistiDg of 8ated switeh 470 and transfer function blocks 486 and 512, or through the serial path consisting of gated switch 472 and transfer function blocks 4~8 and 514, with the transfer function blocks in each path modifyin~ the signal in tbe manncr previously dtscribed. The signal AES
on line 6~6 is subtracted from tbe ~i~nal SE!i on line 604 by summer 392 and is provided 039.264as the draft crror signal DES on line 434 to amplifier 432 Or thc mi~er block 220. Signal DES is amplificd by thc valuc k, and thercafter flows through thc scrial signa1 path consisting of summcr 3g4, block 210, summcr 386 snd block 212 to thc PWM gcnerator circuit 180, whcrc it cmcrgcs as part of signal VCS.

Thc ma~imum 1Owcr ratc control block 210 providcs a means for limiting thc rate at which thc clcctrohydraulic valve 62 may lowcr the rods 146 of hydraulic cylinders 54 and, accordiDgly, thc implcmcnt 57. Spccifically, the lower rate signal LR on linc 118 from pot 98 is providcd as a second input to thc control block 210 and its value dctermines the ma~cimum ratc at which thc hitch will be allowcd to be lowcred. The transfer function implemcnted by block 210 is shown in detail in Figure 10. Slanted lincs 620 and 622 have a unity slope, whilc lincs 624 and 626 are substantially horizontal. Hcavy lines 620 and 624 represent the transfcr function of block 210 whcn thc lowcr ratc pot 98is ina firstposition LRI near thefull CWWsetting of pot 96, whilc thc light lines 622 and 626 as wcll as heavy line 620 represent thc transfcr function of block 210 ~vhcn thc pot 98 is in a sccond position LR2 war thc full CW sctting of pot 98. By adjusting the load ratc pot 98, thcn, the operator can eontrol the rate at which the control system 200 will cause the implcmcnt 57 to bc loYvered as dcsircd. This provcs particularly uscful when thc opcrator is opcrating ncar thc power limit of thc tractor, since thc opcrator ean reduce the lower rate to provide furthcr assurance that the implcmcnt S7 will not stall out or dig in thc tractor. Also, lower ratc pot 98 helps a~surc tbat thc implcmcnt will not be dropped too quickly wh~n the operator, after raising the implement to make a 180 degrec turn in thc field, lowers thc implement in prcparation for rcsumin8 cultivation or plowin8-The valvc compcnsatioD module 2n2 compensatcs for thenonlinearities in the opcration of the electrohydraulic valve 62, which may significantly change with the weight of the implement. Figures llA and 11~ are graphs of thc performance of a Dync~/Rivett Z4900 Serics clectrohydraulic proportional three-way valve used in test;ng prototypes of the control system of the present invention on a Stciger tractor of the type shown in Figurc 1. Figurc 1 IA shows the percent flow output as a function of the avcrage DC volts input by signal VCS to valve 62. Curvcs 630,632 39.264 and 634 graphically illustrate how the percent flow significantly decreases as the hydraulic pressure increases from S00 PSI, to IS00 PSI, and to 2000 PSI. Similarly in Figure 1 IB, thc percent hydraulic flow output by valve 62 is shown as a function of the average DC volts input by signal VCS to the valve 62. Specifically curves 640, 642 and 644 show that the percent hydraulic flow increases as the hydraulic pressure created by gravity bearing upon thc implcment 57 and movable portions of hitch 46 increases.
Thus, it will be appreciated that the changing hydraulic pressure introduces considerable nonlinearity into the performance of valvc 62 io terms Or hydraulic flow rates. To compensate for these nonlincarities, thc control system 200 of the prcscnt invention employs a valve compensation module 212, whose transfer function is illustrated in Figure 12. Notc that the valve 62 does not produce any flow in either dircction until the signal VCS reaches an absolute magnitude of somcwhat larger than two volts. This nonlinearity is taken care of by thc addition of an appropriatc voltage Icvcl (positive or negative) as shown in Figure 12. Also, for c~amplc, when implement S7 has a weight sufficicnt to require 500 PSI hydraulic pressurc (or thereabouts) to ra;sc it, thc block 212 provides a positive and negative linear gain represcntcd by lines 650 and 652, respectively in Figure 12. When the weight of implement 57 requircs about 2000 PSI to raise the implement, block 212 provides positive and negative gain as indicated by lines 656 and 658. The control system 200 learns of the magnitudc of thc hydraulic prcssurc rcquircd to raisc tbc implemcnt through thc Icarn modc control 214 which is a sequence of stcps programmcd into the controllcs 60.

Thc Icarn modc control 214 prefcrably operates in the following manncr. With thc tractor 30 at rcst (not moving) and warmcd up so that thc hydraulic fluid is within a prcferred opcrating tcmperaturc range, and with the front portion 148 of the implcmcnt 57 raiscd so that thc plows are out of the ,~round, the operator pushes the Icarn button 88. Nc~t, thc module 214 provides a command on line 660 which causcs the P~VM gcncrator 180 to ~c~in to slowly ramp up the VCS signal with positivc PWM
curtcnt at a predetcrmined ratc. ~hc controller 60 thcn waits until a predetermincd rate of movemcnt of the hitch is dctcctcd by hitch pot 64, as indicated by the actual hitch position signa1 on line ~6. Specifically, thc controllcr 60 looks for a certain incrementa1 upward movcn~ent oî the HPA signal within a predctcr~nincd brief period 39.264 of time such as scveral tenths of a second. In this manner, the controllcr 60 obtains a relativc indication of the weight of thc implcmcnt by knowiQg the averagc voltage valuc of the VCS signal bein8 applied to the valve that produced the movement of the implement. Thereafter, the controller 60 determines from a stored tab1e of c~perimentally determined values what the preferred 8ain or slope for raising (such as slopc of linc 6S0 or 656) and the prefcrred 8ain or slope for lowering (such as thc slope of linc 652 or 658) should be. Also, the controller 60 records the initial voltage at which thc hitch first be8an to pcrccptibly movc (at some very minimum threshold rate) and uses that voltage value as the off-set in the positive directioo. Similarly, the controller 60 thereafter begins to slowly ramp up the VCS signal with a ncgative PWM current signal at a predetermined }ate, looks for a predetermined rate of downward movement of the hitch, and records and uses that average voltage value of thc signal VCS as the offsct in the negativc dircction. For c~tample, if for 2000 PSI hydraulic pressurc, the controller 60 rlotcs that tbc valvc 62 first bcgins to raisc thc implement at the minimum threshold rate at 2.1 volts, then that voltagc is uscd as thc point 666 at which thc line 656 intersects the vertical a~is in Figure 12. Similarly, if at S00 PSI hydraulic prcssure thc controller 60 notcs that this minimum threshold Icvcl is achicvcd at 2.25 volts whcn raising thc implcmcDt theD thc line 6S0 will bc madc to interscct thc vcrtical a~is in graph of Figure 12 at that voltagc, which corrcsponds to point 660. Likcwisc, if thcse minimum lowcr rate thrcshold valucs of signal VCS for S00 PSI and 2000 PSI hydraulic prcssures arc -2.3 volts and -2 '~ volts respectively, the poiDts 662 and 668 OD the vertical a~is of thc graph ;D Figurc 12 are sclected as the start points for lines 652 and 6~8, respectively. Every time the tractor operator places a new implement upon the hitch 46, the foregoing sequence should be peJformed so that the transfer function characteristics of block 212 can bc tailored to the weight of the implemcnt. ID light of the forcgoing description, those skilled ;D thc art should apprcciate that the valve compcnsatioD module 212 and lcarn modc control 214 provide a technique for grcatly improving thc overall pcrformance of the automatic fccdback control systcm of thc present invcntion bycffectively climinatin~ the nonlincar hydraulic pcrformancc of valve 62 obscrvedbetween implcments of different weight.

-~9-039.264 The purpose of the slip-sensing control 208 is to send a suitable command to raise the implement whenever the slippage becomes e~cessive, so as to avoid unwanted digging in the implement or unnecessarily slowing the movement of the tractor until a manua1 adjustment of the hiteh is made by the operator. The generation of such a signal to raise the implement begins with the subtraction of the scaled value of indicated ground speed si8nal on line 680 from the scaled value of actual ground speed on line 682 by summer 398. Summer 398 outputs the difference on linc 684 and it is received as input ~A~ of div;der S40. Input B of divider S40 is the scaled value of - indicated ground speed OD line 68Q. The divider block 540 divides the value provided at input A by the value provided by input B. The resulting quoticnt is output on line 542 as the slip (SL) signal and is an input to block ~20, whose transfer function is illustrated in the graph of Figure 9. The block 520 also receives as an input signal the conditioned and riltered slip rate signal from line 690. The block S20 provides DO
output until si~nal SLP reaches a certain minimum threshold speeified by the setting of slip-rate pot 100, as indicated by the magnitude of the signal on line 690. Once the signal SLP e%ceeds this threshold, the output of block 520, which for convenience may be called the slip compensation signal SLC, rises linearly with i~creasing magnitude of the input signal, as represented in Figure 9. In Figure 9, point 694 represents the minimum threshold when the pot 100 is set at a first setting SRl near its full CCW position, while point 696 represents the minimum threshold level when pot 100 is in a hi8her setting SR2 near its fu11 CW position. Heavy line 692 and light 1ine 698 show the resulting linear increase in the slip compensation signal when pot 100 is in the setting SRJ and SR2, respectively. The slope of line 6g2 (or line 698) corresponds to the gain of the slip-sensing control loop 208. This slope and hence the ~ain of control 208 may be optimized for a given tractor/transmission/wheel combination by a few simple e~periments in the field which those skilled in the art would kDow how to perform. From the foregoing descrip~ion those ill thc art will appreciate that the slip-sensing control 208 provides a means by which a slip compensa~ion signal will be provided through summer 386 and valve compensation module 212 and the PWM generator circuit 180 to the electrohydraulic valve so as to raise the implement in proportion to the amount of slip in e~cess of the slip rate settins of pot 100. The slip rate pot 100 provides the operator with the ability to adjust the slip rate sett}ng to wbat be considers an optimum va1uc.
-3~

132~7 039.264 This also provides a mechanism by whjch the opcrator can compensatc for different wheel configurations that hc may cmploy with the tractor. Also, it will be appreciated that the slip-sensing control is of 8reat benefit to the draft control system of the present invention, since it provides a remedy for the situation where because of wheel slippage, the draft feedback control 204 would no longer be able to correlate the draft or load forces bein8 produced by the implement with the deviation of the actual engine speed from thc tar8et point speed.

The upshift/downshift control 206 provides a further means for the contro1 system 200 to avoid or compensate for possible problems with engine stall out that may occur, especial1y when engine speed is below the tar8et point speed by an unacceptable amount and is not recovering, but instead is continuing to decreasc. The downshift portion 700 of control 206 is provided by blocks 534, 535 and S52, while the upshift portion 702 of control 206 is provided by summcr 394 aDd blocks 5~0, S32 and 550. In downshift portion 700, the block 534 receivcs as an input the signal ABT, which indicates the amount by which the actual cnginc speçd is below the target point speed.
The block 536 receives as an input the output 478 of 8ated switch 472, which is the en~ine speed rate signal ESR when the actual engine speed is below thc target point speed. Block S36 produces an output when signal ESR falls below a predetcrmined value established by the location of l;DC 708 along the negative portion of ths horizontal a%is of the transfer function shown in block S36. The position of line 708 is sclected to reflcct a predetermined rate of engine decclcration which indicates that thc actual speed of the engine 32 is likely to continue to decrease rathcr than recover. Whcn the signal on line 706 is in a logical one state and the signal ABT reaches a predetermined positivc thrcshold value determined by the location of line scgment 710 along the horizontal a~is of the ~ransfer function of ~lock 534, thc block S34 will produce a logical one output on line 712. When single shot block S52 detects a logical onc va1ue on linc 712, it produces a momentary positive pulsc on its output 556 which is suitably conditioned by output buffer/amplifier 716 and thereafter delivered to the transmission control1er 31, which commands the transmission 33 to downshift into the ne~t lower 8ear. This provides more torque and allows the en~ine speed to rccover, thus preventing en~ine stall-out.

1~2~97 039.264 In upshift portion 702, summer 394 receives as outputs the buffered, filtered and compeosa~ed desired en8ine speed signal ESD' aDd the buffered and scaled engine speed signal ESA', which is subtracted by summer 394 from signal ESD'. This difference represents the amount by which the actual engine speed is below the desired engine speed. Bloek 532 receives as an input the output of gated switch 470, which represents the si8nal ESR (when the aetual engine speed is above the desired engine speed). Like in bloek 536, block 532 will produce a logical one output on 1ine 724 whea the input value e~eeeds a predetermined threshold indicated by the line 726 in the transfer function graph within block S32. When block 530 receives a logical one input on line 724 and wheD the maRnitude of the signal bein~ input by line 720 is below a minimum threshold indicated by vertical line 722 of the transfer function graph within block 530, block S30 will produce a lo~ical one output on line 553. In other words, block 530 will produce a logical one output on line 553 when the actual eogine speed is above the desired engine speed and the engine acceleration is iDcreasing above the predetermined threshold indicated by line 726. When singie shot block SS0 perceives a lo~ical one value on line S53 it produces a momentary logical one pulse on its output 5S4, which after ~uitable buffering ~y output ampl;fier 730 is sent as an upshift signal to transmission controller 31, which causes the transmission 33 to upshift into the ne~t higher 8ear. Thus, the upshift/downshift co~trol 206 provides sutomatic means for downshifting and upshifting of the transmission 33, so ~hat the operator does not have to manually upshift ~nd downshift his transmissioD to take care of minor chan8es and conditions sueh as are produced when drivin~ over a small rise or ridge.

The foregoing detailed description sllows that the preferred embodiments of the present invention are well suited to fulfill the ob~eets above stated.
It is recognized that those io the art may make various modifications or additions to the preferred embodiments chosen to illustrate the ~resent invention without departing from the spirit and the proper scope of the present i~vention. For e%smple, the automatic feedback control system of the preSeDt invention may be employed in dozers, road graders and other suitable off-road construction industry vehieles and off-road agricultural vehicles. In the case of dozers and road graders, for instance, the ~round-eDgaging implement whose elevatioD is to ~e re~ulated in accordance with the sensed 39.264 en8ine lug-down would typically be the blade or scraper. Accordingly, it is to bc understood that the protection sou~ht and to be afforded hereby should be deemed to e~tend to the subject matter defined by the appended claims, including all fair equivalents thereof.

Claims

1. In a self-propelled vehicle having an engine, ground-engaging traction means for moving the vehicle relative to the ground, and connecting means for attaching a ground-penetrating implement thereto and actuating means for adjusting the elevation of at least a first portion of the implement to vary the ground penetration thereof in response to at least one control signal applied to the actuating means, an automatic feedback control system comprising:
first sensing means for generating a speed parameter signal representative of the actual speed of the engine;
first input means for producing a no-load engine speed signal corresponding to a no-load engine speed;
speed input means for producing a target speed signal which corresponds to a target engine speed which is less than the no-load engine speed;
first difference means for generating a speed error signal representing a difference between the speed parameter signal and the target speed signal;
and first control means for operating the actuating means to adjust the elevation of the first portion of the implement at least partially in response to the speed error signal;
second sensing means for generating an elevation parameter signal representative of the elevation of at least the first portion of the implement relative to the vehicle;
third input means for producing a target elevation signal;
second difference means for generating an elevation error signal representing a difference between the elevation parameter signal and the desired elevation signal;
second control means for operating the actuating means to adjust the elevation of the first portion of the implement at least partially in response to the elevation error signal;
mixing means for adjusting the relative influence that the first control means and the second control means has upon the operation of the actuating means; and wherein the mixing means is responsive to the magnitude of the difference between the no-load engine speed and the target engine speed and as the magnitude increases, the mixing means decreases the relative influence of the elevation error signal and increases the relative influence of the speed error signal.

2. A control system as in claim 1, further comprising:
first rate-of-change means for producing an engine acceleration parameter signal representative of the rate at which the actual speed of the engine is changing;
acceleration feedback control means for providing at least a portion of the acceleration parameter signal to the first control means; and wherein the first control means includes means for combining the provided portion of the acceleration parameter signal with at least a portion of the engine speed error signal.

3. A control system as in claim 2, wherein the acceleration feedback control means includes (a) means for determining when the engine speed parameter signal is greater than the target speed signal, (b) means for determining when the engine speed parameter signal is less than the target speed signal, (c) means for providing a first gain in the acceleration feedback control means when the engine speed parameter signal is greater than the target speed signal, and (d) means for providing a second gain in the acceleration feedback control means when the engine speed parameter signal is less than the target speed signal, with the second gain being greater than the first gain.

4. A control system as in claim 1, wherein the control system further comprises:
downshift signal means for providing a downshift command signal to the vehicle only after the actual engine speed falls to a predetermined downshift threshold speed.

5. A control system as in claim 4, wherein the predetermined minimum speed is computed by calculations based at least in part upon the target speed.

6. A control system as in claim 5, wherein the downshift signal means includes means for determining the rate of change of engine speed, and produces a downshift command signal only after the rate of change of engine speed indicates that engine speed is decreasing at a preselected minimum rate.

7. A control system as in claim 5, further comprising:
upshift signal means for providing an upshift signal to the vehicle only after the engine speed has increased to a predetermined upshift threshold speed.

8. A control system as in claim 7, wherein the upshift signal means includes means for establishing predetermined upshift threshold speed based at least in part upon the no-load engine speed.

9. A control system as in claim 8, wherein:
the downshift signal means includes means for determining the rate of change of engine speed, and produces the downshift command signal only after the rate of change of engine speed, and produces the downshift command signal only after the rate of change of engine speed indicates that engine speed is decreasing by at least a first preselected minimum rate, and the upshift means include means for determining the rate of change of engine speed, and produces the upshift command signal only after the rate of change of engine speed indicates that engine speed is increasing by at least a second preselected minimum rate.

10. A control system as in claim 1, further comprising:
first compensation means for adjusting the first control means to compensate for nonlinear operating characteristics of the actuating means.

11. A control system as in claim 10, further comprising:
second compensation means for adjusting the first control means to compensate for the weight of the implement applied to the actuating means at a selected interval of time.

12. A control system as in claim 1, wherein the first sensing means includes a sensor for sensing the relative position of an actuator means affecting movement of a throttle of the engine, and means for calibrating the first sensing means under no-load engine conditions by associating a predetermined number of output values from the sensor with respective ones of a plurality of distinct positions of the actuator means which cause the engine to operate at different speeds and storing a value corresponding to the actual engine speed for each of the distinct positions of the actuator means.

13. A control system as in claim 12, wherein the sensor is a potentiometer and the actuator means is a throttle lever rotatable about a pivot point.

14. A control system as in claim 1, wherein the second input means includes means for producing a desired draft load signal which corresponds to a desired reduction in engine speed when the engine is under load, and first difference means for generating the target speed signal from the no-load engine speed signal and the desired draft load signal.

15. In a self-propelled vehicle having an engine, ground-engaging traction means for moving the vehicle relative to the ground, and connecting means for attaching a ground-penetrating implement thereto and actuating means for adjusting the elevation of at least a first portion of the implement to vary the ground penetration thereof in response to at least one control signal applied to the actuating means, an automatic feedback control system comprising:
first sensing means for generating a speed parameter signal representative of the actual speed of the engine;
first input means for producing a no-load engine speed signal corresponding to an engine speed when the vehicle is not under load;
second input means for producing a draft load signal expressed as a function of reduction in the speed of the engine when under load;
first difference means for generating a target speed signal from the no-load engine speed signal and draft load signal;
second difference means for generating a speed error signal representing a difference between the speed parameter signal and the target speed signal;
first control means for operating the actuating means to adjust the elevation of the first portion of the implement at least partially in response to the speed error signal;
third input means for adjusting and limiting the rate at which the actuating means lowers the implement;
means for determining a slip signal representing a difference between a true ground speed signal and an indicated ground speed signal;
first comparison means of producing an excessive slip signal when the slip signal indicates the amount of slip has passed a predetermined threshold level;
second control means for operating the actuating means to raise the first portion of the implement in response to the excessive slip signal;
first rate-of-change means for producing an acceleration parameter signal representative of the rate at which the actual speed of the engine is changing; and acceleration feedback control means for providing at least a portion of the acceleration parameter signal to the first control means;
wherein the first control means includes means for combining at least a portion of the acceleration parameter signal with at least a portion of the engine speed error signal; and wherein the acceleration feedback control means includes (a) means for determining when the engine speed parameter signal is greater than the target speed signal, (b) means for determining when the engine speed parameter signal is less than the target speed signal, (c) means for providing a first gain in the acceleration feedback control means when the engine speed parameter signal is greater than the target speed signal, and (d) means for providing a second gain in the acceleration feedback control means when the engine speed parameter signal is less than the target speed signal.